peerspeak/docs/screenshare-audio-exclusion-plan.md

# Design v3: whole-desktop screen-share audio without self-echo

**Status:** v3.3 — round-6 blockers resolved; **awaiting round-7 ratification** (§14).
**Date:** 2026-07-21 (v1: 07-19 · v2: 07-20 · Option C 07-20 · v3.1 r4 · v3.2 r5 · v3.3 r6)
**Origin:** Joe's suggestion — "whitelist all audio except audio coming from peerspeak."
**Scope:** a new capture mode in pixelpass (`src/host/pipeline.rs`, `src/host/audio.rs`),
playback tagging + AEC-identity export + teardown-ordering invariants in peerspeak.

**Why v3 is a rewrite, not a patch.** v1 and v2 both described a design that *moves*
desktop streams onto a capture sink. That design is dead. v2's §5.1 adopted **Option C
(copy via a second owned link)** after a measured feasibility spike, and the AEC
identity gate — the one thing both reviewers agreed nothing could go ahead of — has
since passed. Roughly two thirds of v2 described problems Option C does not have.
Carrying that text forward as "retained for the record" made the live plan unreadable.
v1/v2 remain in git history at `88ad5a0` and `10203e1`.

**Review history**
| Round | Artifact | Verdict |
| --- | --- | --- |
| v1 review | `~/Documents/handoff-docs/Codex/peerspeak/review-2026-07-19-audio-exclusion-design.md` | not sound as written; 4 blockers, all verified correct |
| v2 review | `…/review-2026-07-20-audio-exclusion-design-v2.md` | blocked; produced Option C |
| fan-out spike | `~/Documents/handoff-docs/Claude/peerspeak/fanout-spike-results-2026-07-20.md` + Codex rounds 3/4 | **Option C adopted**, ratified |
| AEC identity gate | `~/Documents/handoff-docs/Claude/peerspeak/aec-playback-leg-identity-2026-07-20.md` | **🟢 gate passed**, both models agree after 2 adversarial rounds |

---

## 1. The problem

A sharer using whole-desktop audio sends their entire output mix to viewers. That mix
necessarily includes peerspeak's own playback — remote peers' voices, remote screen-share
audio — so viewers hear themselves.

`--strict-audio` solved this for *per-app* mode by refusing to mirror the desktop at all.
Whole-desktop mode has no equivalent, so a sharer today chooses between echo and sharing
exactly one app's audio. Joe's ask is the missing third option: **share the desktop,
minus peerspeak.**

## 2. Framing

PipeWire has no capture-side filter. A monitor port carries an already-summed signal;
once peerspeak's output is in that mix it cannot be subtracted out. What PipeWire does
provide is **graph topology**: a node's output ports may link to more than one consumer.

So the feature is built by constructing a sink that only *eligible* streams feed, and
capturing that sink's monitor. Same observable behaviour as a filter, entirely different
mechanism.

This is **not** echo cancellation. `src/audio/echo_cancel.rs` addresses the microphone
path, where coupling is acoustic and needs adaptive cancellation. Here the signals never
need to be summed at all.

**Fidelity is not claimed.** The *membership* of the constructed mix is exact — peerspeak's
audio is never summed in. The *delivered audio* is not lossless: pixelpass downmixes to
48 kHz stereo and AAC-encodes at 128 kbps (`pixelpass/src/host/pipeline.rs:303-324`).
Adding a second link also makes the source node participate in a second format/buffer
negotiation, which can perturb it even though the original link survives (§9.3).

## 3. What exists today

`setup_audio` (`pixelpass/src/host/pipeline.rs:123-142`) activates `Routing` **only** when
`--app` is set or `PIXELPASS_AUDIO_VIA_NULL_SINK` is set. peerspeak's no-app argv is
`--host --output json` (`peerspeak/src/screenshare/mod.rs:135-165`), so normal
whole-desktop capture **bypasses `Routing` entirely** and hands the real default monitor
to `pulsesrc`. This feature is therefore a genuinely new mode, not an inverted predicate.

`Routing::start` (`pixelpass/src/host/audio.rs:65-212`) today provides:

| Piece | Condition |
| --- | --- |
| `module-null-sink pixelpass_capture_<pid>` | always, when `Routing` runs at all |
| `module-loopback @DEFAULT_SINK@.monitor → capture` | skipped only when `--app` **and** `--strict-audio` |
| `StreamRouter` (libpipewire thread) | only when `opts.app` is `Some` |
| local-monitor `capture.monitor → @DEFAULT_SINK@` | on `FirstRoutedStream`, unloaded on `LastRoutedStreamGone` |

Under Option C the null sink is reused, the monitor loopback is never loaded in this
mode, the local monitor is not needed at all, and `StreamRouter` is replaced by a
link manager rather than extended. Two existing defects in that file are inherited and
must be dealt with as prerequisites (§10).

## 4. Architecture — Option C: copy, don't move

pixelpass creates a *second*, pixelpass-owned link from each eligible playback stream's
output ports to the capture sink, and leaves the stream's existing route untouched.

```
  app output ──────────────────────► existing hardware / filter path (UNTOUCHED)
       └── pixelpass-owned link ────► pixelpass_capture_<pid> ──► gst pulsesrc ──► viewers

  peerspeak-owned playback ────────► speakers only   (never linked to capture)
  AEC playback leg ────────────────► speakers only   (never linked to capture)
```

### 4.1 Why this beats the move-based design

| Problem under move (Option A) | Under copy |
| --- | --- |
| local-monitor loopback needed to keep the sharer hearing audio | not needed — audio never leaves the speakers |
| added playback latency for the sharer | gone |
| output-device switch mid-share strands the desktop | gone — the app's own route is never touched |
| prior `target.object` capture/restore | gone — nothing is retargeted |
| pavucontrol conflicts | gone |
| two concurrent hosts fight over `target.object` | gone — links are independent per host |
| SIGKILL strands the whole desktop on an orphan sink | gone — see §4.2 |

**The decisive measurement (spike E2): destroying the capture sink mid-share left the
application playing to its speakers undisturbed.** Capture-side failure degrades to "not
captured," never to "the user's audio is broken." That is Option C's entire case, and it
is the reason a broader eligibility predicate is now acceptable (§6).

### 4.2 Ownership of links is the safety mechanism

In the Rust `pipewire` crate, **dropping a `Link` proxy destroys the object.** So:

- Links are created **non-lingering** and their proxies are **retained** for the life of
  the share.
- Process death (including SIGKILL) destroys the connection, which destroys the links.
  Measured: non-lingering links die on SIGKILL of the owner.
- A stream is counted as captured only once **every** required link reaches `ACTIVE`.
- On any link failure: leave the original route alone, report the stream as unsupported.
  Never fall back to "capture the default monitor instead" — that fallback *is* the echo.

⚠️ **Rig gotchas that constrain how this is built and tested** (learned the hard way):
`pw-link --props object.linger=false` and `pw-cli create-link <props>` both **ignore**
linger and force it on; only `pw-link -m` yields a non-lingering link. `pw-cli
create-object` does not exist in 1.6.8 — the verb is `create-link <node> <port> <node>
<port>`. And links do **not** self-restore after a node or sink is recreated: a *live
owner* must re-link. That re-link loop is the pattern pixelpass has to implement, and
it is also what makes the daemon-restart case (§12) untested rather than free.

## 5. Ownership: identifying peerspeak's own audio

Every peerspeak-owned playback path is either a stream peerspeak itself creates, a
`Command::new(…)` spawn, or a pactl-loaded module. Three mechanisms cover all three.

### 5.1 Env-inherited tag, for spawned children — mechanism ✅ MEASURED, not yet built

⚠️ **This table lists IMPLEMENTATION SITES, not current behaviour.** None of these sites
tags anything today: `pipewire_impl.rs:374-388` sets no `peerspeak.owned` prop, and
neither the mpv/VLC spawn (`screenshare/mod.rs:768-775`) nor the notification spawn
(`notify.rs:265-272`) sets any env. The *mechanism* is measured working (below); the
*wiring* is work.

| Owner | Site | Mechanism to add |
| --- | --- | --- |
| native call playback | `src/audio/pipewire_impl.rs:374-388` | add the tag to the stream's property dict |
| mpv / VLC | `src/screenshare/mod.rs:768-775` | `.env("PULSE_PROP", …)` + `PIPEWIRE_PROPS` on the `Command` |
| `pw-play` / `paplay` / `aplay` | `src/notify.rs:265-272` | same |

Exact strings to be pinned at implementation time (§11), and each site needs a test:
native props asserted on the constructed dict, child sites asserted on the `Command` env.

Measured 2026-07-20 on this box (PipeWire 1.6.8): `peerspeak.owned=1` reached the graph
node for **paplay ✅, mpv ✅, VLC ✅**. `PULSE_PROP` and `PULSE_PROP_OVERRIDE` are both
present in `/usr/lib/pulseaudio/libpulsecommon-17.0.so`. (Codex asserted this mechanism
did not exist; it was disproved empirically. VLC has no native PipeWire aout on this box,
which is true and irrelevant — its Pulse path honours `PULSE_PROP`.)

**The tag is a correctness mechanism, not a security boundary.** Any same-user client can
set `peerspeak.owned=1` and opt itself out of capture, and a child can sanitize its own
environment. PipeWire is explicit that only `pipewire.*` properties are usable for
security decisions. Acceptable here: the threat model is "don't echo the user's own call
back at them," not "defend against a hostile local process." Stated, not assumed.

⚠️ **Known leak: a stable boolean is inherited by grandchildren.** Anything mpv or VLC
spawns is exempted too. Accepted for now; revisit if it bites.

### 5.2 The AEC playback leg — 🟢 gate passed, with five load-bearing corrections

`module-echo-cancel` is loaded via pactl (`peerspeak/src/audio/echo_cancel.rs:83-94`), so
its playback leg lives inside `pipewire-pulse` and **cannot inherit an env tag.** It is a
`Stream/Output/Audio` node linked straight to the speakers, so a broad selector *will*
pick it up unless explicitly excluded — and that is not a nicety:

> **Measured (3 arms, links verified in-graph before measuring, recorded via
> `parec -d <sink>.monitor` — the production path):** naive fan-out that includes the AEC
> leg copies remote-call audio into the share at **≈desktop level** (−45.0 dB vs the
> −44.5 dB desktop tone). Exact exclusion produces **−83.3 dB, matching the control floor
> to 0.1 dB.**

**What `module-echo-cancel` actually creates (RESULT 1):** four nodes, not two —
`Audio/Sink peerspeak_ec_sink.<pid>`, `Audio/Source peerspeak_ec_source.<pid>`,
`Stream/Output/Audio **echo-cancel-playback**` (passive, virtual), and
`Stream/Input/Audio echo-cancel-capture`. Codex's "Pulse-compat exposes only
`sink_properties`" concern is real about the *module args* and irrelevant to the *graph
props*.

**The identity (RESULT 2):** all four nodes carry `pulse.module.id` equal, byte for byte,
to the module index `pactl load-module` returned — the value peerspeak already stores in
`EchoCancelGuard::module_index` (`echo_cancel.rs:106-112`, a `String` validated as `u64`).
No PID guesswork, no name matching, no leg correlation.

```
exclude every node where pulse.module.id == <the index pactl returned to the live guard>
```

**⚠️ The five corrections. These are the design, not footnotes:**

1. **This is not an "identity contract."** It is an exact correlation *observed on
   PipeWire 1.6.8*, not a documented API. No PipeWire source is installed on this box and
   neither model had network, so whether the property is the *mechanism* pipewire-pulse
   uses to reap module-created objects or a cosmetic mirror is **UNRESOLVED**.
   ⇒ **Runtime-validate and fail closed** (§5.3).
2. **"Has any `pulse.module.id`" is REJECTED as an exclusion rule.** Tunnel, RTP and
   loopback modules may be the *only* carrier of audio the user legitimately wants
   shared. **Exact equality with the owned live index — nothing weaker.**
3. **Module index and `node.link-group` are REUSED verbatim across unload/reload** (both
   came back `536870919` / `echo-cancel-1974-13`), and **node IDs are recycled *and
   reassigned across legs*** — id 136 was the playback leg on load 1 and the capture leg
   on load 2. ⇒ **Never cache a node id. Never assume leg order. Never cache the module
   index across an unload** — it is only trustworthy as "the index for the currently-live
   guard," so pixelpass must be (re)told on every load.
4. **The `echo-cancel-` group prefix is hazard *detection*, not ownership.** Neither it
   nor the fixed node name `echo-cancel-playback` identifies *peerspeak's* instance. A
   foreign or second AEC is a product-policy question (§5.4). The better long-term answer
   is the **native** PipeWire AEC module, which exposes `playback.props` and would let us
   stamp our own random token directly on the playback leg.
5. **PID — the whole muddle, finally resolved** (third measurement, 2026-07-21, settles
   four rounds of contradictory claims). Two *different properties on two different
   object types* were being conflated:

   | property | lives on | value for Pulse-emulated clients |
   | --- | --- | --- |
   | `application.process.id` | the **Node** | the app's **own** PID (Firefox `11114`, paplay's own) |
   | `pipewire.sec.pid` | the **Client** | **pipewire-pulse's** PID — `2541` for *every* Pulse client on this box (Firefox, Steam, KDE Connect, libcanberra all identical) |

   Module-created streams (the AEC) are the exception on the *node* side: their
   `application.process.id` is pipewire-pulse's (`1974` at the time of that run), because
   pipewire-pulse genuinely is the client. Earlier notes claiming `pipewire.sec.pid` was
   "absent on these nodes" were right — it is absent from **nodes** because it is a
   **Client** property. **PID-based AEC exclusion stays unusable**; PID is a fallback
   hint, never identity. `client.id` — *not* PID — is the usable per-app correlation key
   (§6.1 edge type 3).

Parse defensively: `pulse.module.id` renders as a JSON **number** in `pw-dump` but SPA
props are strings. `536870919 = 0x20000007`; pipewire-pulse indices start at `0x20000000`
so they fit u32 — but this sits right next to the `object.serial` u32-truncation bug
(§10.1), so compare as strings or as `u64`, never as `u32`.

### 5.3 Fail closed — validation and revocation

Because the identity is an observed correlation rather than a contract, pixelpass must
**verify it at runtime and refuse to run the mode when it cannot.** A one-shot
"enumerate at start" check is *not* good enough — it races, in both directions:

- peerspeak's `enable()` returns once `wait_for_nodes` sees the virtual **source and
  sink** by name (`echo_cancel.rs:132-158`, polling `pactl list sources/sinks short`).
  It does **not** wait for the playback `Stream/Output/Audio` hazard leg, which is the
  node we actually need to exclude.
- pixelpass's capture spawns **lazily, on first viewer** (`pixelpass/src/host/mod.rs:300-304`),
  so enumeration happens at a moment peerspeak does not control, and a refusal surfaces
  *after* the ticket has been handed out.

**So validation is a bounded state machine, not a check:**

```
NotConfigured  ──(--aec=off)──────────────► fan-out proceeds, no AEC exclusion
Validating     ──(--aec=pulse-module:<i>)─► enumerate after a registry sync barrier;
                                            wait up to a bounded deadline for ≥1 node
                                            with pulse.module.id == i
Validated      ──► fan-out permitted, excluding that identity transitively (§6.1)
Failed         ──► deadline expired, identity never observed  ⇒ NO FAN-OUT
Revoked        ──► the live module identity disappeared        ⇒ STOP FAN-OUT NOW
```

- **No fan-out occurs in `Validating`.** Silence is the safe direction; echo is not.
- **`Failed` is fail-closed**: report a capability failure, do not silently share.
  Fall back to a mode with no echo risk, or an explicit user override.
- **Revocation is loss of the live *module* identity** — *all* nodes bearing the index
  gone — not the transient absence of one playback leg, which can cork or relink. Getting
  this wrong turns a normal cork into a spurious share-wide audio stop.
- On `Revoked`, **drop the link proxies**; do not keep the numeric index and hope. The
  index is reused (§5.2 correction 3), so a retained stale index can alias onto an
  unrelated future module.
- **Never** infer exclusion from `node.name == "echo-cancel-playback"` alone — it is a
  fixed, non-unique, trivially spoofable string. Usable only as belt-and-braces *after*
  the exact match, or as foreign-instance hazard detection.

### 5.4 Foreign or second AEC instances — a product decision

If a node matching `node.link-group` prefix `echo-cancel-` exists that is **not** ours,
peerspeak is not the only echo canceller on the box. Options: (a) fail closed — refuse
the mode; (b) warn and exclude all `echo-cancel-*` groups, accepting that a legitimate
unrelated AEC's output silently won't be shared. **Recommendation: (b) with a visible
warning** — the failure it prevents (echo) is worse than the failure it causes (one
app's audio missing), and it matches §6's overall posture. **Open decision D3 (§13).**

## 6. Eligibility — a broad guarded selector

Under a *move* design, mis-selection rewired the user's desktop, which forced a narrow
allowlist. Under copy, mis-selection costs at most one stream's capture — so the narrow
allowlist is out. But "fan out everything" is still wrong: it recreates self-echo, it
recreates call echo, it can build cycles, and a second link participates in format/buffer
negotiation.

### 6.1 ⚠️ Eligibility is a GRAPH property, not a node property

**This is the single biggest correction in round 4, and it is reachable on the target
machine, not hypothetical.**

A node-local property check is unsound, because **exclusion does not propagate downstream.**
If any node re-emits audio it received, the re-emitting node is a fresh, *untagged*
`Stream/Output/Audio` carrying the mix — including peerspeak's playback and the AEC leg,
both of which were correctly excluded one hop earlier.

The canonical instance: **EasyEffects.** When it is running, applications play into
`ee_sink` and EasyEffects emits a single combined output stream to the hardware. That
output leg has no `peerspeak.owned` tag and no `pulse.module.id` matching our AEC, so
**both §5.1 and §5.2 pass it**, and fanning it out re-injects the entire call into the
share at full level — the exact bug this feature exists to prevent, arriving through the
front door. ⚠️ **`easyeffects` is installed on this machine** (`pacman -Qq easyeffects`),
and `libpipewire-module-filter-chain.so` is present. It happened not to be running during
the spike, which is *why* the spike did not catch this. Same shape: combine-sinks,
`module-loopback`, virtual-sink forwarders, tunnel/RTP sinks, DSP bridges, and any
monitor-sourced stream.

**So the predicate is transitive TAINT over the live SIGNAL graph** — and "signal graph"
is doing real work in that sentence, because it is **not** the graph of `Link` objects.

> A node is eligible only if **no** signal path reaches it from a peerspeak-owned node,
> the live AEC identity, or any pixelpass-owned object (ours *or* another host's
> `pixelpass_capture_*`). Unresolvable ancestry is **not** eligible.

⚠️ **A Link-only walk does NOT catch the leak.** Measured on the live graph 2026-07-21
by reproducing the forwarder topology with `module-null-sink` + `module-loopback`
(same shape as EasyEffects, no EasyEffects required):

```
  fabletest_sink:monitor_FL ──Link 87──►  input.loopback-2541-13   (Stream/Input/Audio,  id 109)
                                                    ⋮  ← NO LINK OBJECT EXISTS HERE
  output.loopback-2541-13   ──Link 105─►  alsa_output…analog-stereo (id 56)
   (Stream/Output/Audio, id 86)
```

**There is no `Link` between the forwarder's input leg (109) and its output leg (86).**
Walking upstream from the leaking node 86 over Links alone finds *no inbound links at
all* — a dead end that reads as "clean," and the node gets fanned out. The two legs are
related only by shared properties:

| key | input leg (109) | output leg (86) |
| --- | --- | --- |
| `node.link-group` | `loopback-2541-13` | `loopback-2541-13` |
| `client.id` | `107` | `107` |
| `pulse.module.id` | `536870917` | `536870917` |

So the signal graph needs **three** edge types:

1. **Link edges** — `link.output.node → link.input.node`. ✅ Measured: registry Link
   objects carry all four endpoint props (`link.output.node`, `link.output.port`,
   `link.input.node`, `link.input.port`).
2. **Sink-monitor edges** — ✅ **free at node granularity.** Measured: the monitor
   connection *is* a real `Link` whose output node is the sink node itself
   (`out-node=104 (fabletest_sink) → in-node=109`). So a node-level walk crosses
   `app → sink → monitor-reader` with no synthetic edge. A **port**-granular walk *does*
   need an explicit rule (inputs feed `port.monitor=true` outputs); a node-granular one
   does not. **Use node granularity for taint; use ports only for link creation.**
   ⚠️ **Accepted cost — duplex over-taint.** Node granularity smears taint across
   multi-role nodes: an `Audio/Duplex` node whose playback side is tainted will have its
   capture side treated as tainted too. That is **fail-closed** (over-exclusion, not a
   leak) and is accepted for v1, but it can contradict the "Firefox with a microphone
   stays shareable" promise in §6.1.1 on a duplex device. Port-modelling `Audio/Duplex`
   and unknown-role nodes is the fix if this bites in field testing.
3. **Owner-bridge edges** — the intra-process hop the graph cannot see. This one is
   genuinely hard; see §6.1.2. A single key does **not** work.

### 6.1.2 ⚠️ The owner bridge — no single key works (measured refutation)

**I claimed `client.id` was the fallback bridge key for apps without a `link-group`.
I disproved my own claim; Codex independently reached the same verdict.**

Test: one `gst-launch-1.0 pulsesrc device=fable_src.monitor ! audioconvert ! pulsesink
device=fable_dst` process — one PID, an input leg and an output leg, the exact
OBS/recorder/forwarder shape, and the same framework pixelpass itself uses.

| leg | `client.id` | `node.link-group` | `application.process.id` |
| --- | --- | --- | --- |
| `Stream/Input/Audio` | **209** | null | 20172 |
| `Stream/Output/Audio` | **210** | null | 20172 |

**Two distinct client objects for one process**, both named `gst-launch-1.0`, both with
`pipewire.sec.pid=2541`. So `client.id` bridges *a client connection*, not *an owner* —
an app is free to open one connection per stream, and GStreamer does exactly that. The
earlier "libcanberra appears as clients 74 and 77" observation was the same signal and I
under-weighted it.

**Replacement: a conservative union of owner keys, strongest first**, taking the first
that resolves:

| # | key | scope | notes |
| --- | --- | --- | --- |
| 1 | `node.link-group` | per module/filter instance | precise; set by PipeWire modules (loopback, echo-cancel, filter-chain) |
| 2 | `pulse.module.id` | per pactl module | precise; module-created streams only |
| 3 | `client.id` | per **connection** | correct when an app uses one connection; insufficient alone |
| 4 | `application.process.id` (on the **Node**) | per process | ✅ the only key that bridged the gst-launch legs (`20172` on both) |
| — | else | — | **unresolved ⇒ fail closed** (exclude the output leg) |

⚠️ **Key 4 has a trap that must be coded explicitly.** For *module-created* streams,
`application.process.id` is **pipewire-pulse's own PID** (§5.2 correction 5). Bridging on
it would fuse *every* Pulse module's legs into one owner, so a single tainted module input
would exclude every module-created stream on the box — mass over-exclusion, and exactly
the failure correction 2 warned about (a tunnel/RTP module may be the sole carrier of
audio the user wants shared). **So: never bridge on key 4 when the value equals the
pipewire-pulse PID.** Those cases are already covered precisely by keys 1 and 2.

Note the irony worth recording: PID returns to the design, but in a different *role*.
It is unusable as **identity** ("is this peerspeak's audio?") and workable as
**correlation** ("are these two legs the same app?") — and in the correlation role a
wrong answer fails closed rather than leaking.

### 6.1.1 ⚠️ Bridge taint must be conditional, or it over-excludes badly

The owner bridge must propagate taint **only when the input leg is itself tainted.**
The naive rule "this client has both an input and an output leg ⇒ exclude the output" is
catastrophic: it excludes any app using a microphone. Firefox in a video call has both
legs, and its playback is perfectly shareable.

The correct rule:

1. Taint roots: peerspeak-owned nodes, live AEC identity, pixelpass-owned objects.
2. Taint flows downstream along Link edges (crossing sinks and monitors for free, §6.1
   edge type 2). A sink carrying tainted audio has a **tainted monitor**.
3. A `Stream/Input/Audio` reading a tainted monitor becomes tainted.
4. **Only then** does the owner bridge carry taint to that client's `Stream/Output/Audio`
   legs.

Result, both cases correct:

| scenario | outcome |
| --- | --- |
| Firefox playing music (output leg only) | **eligible** — shared |
| Firefox in a Meet call (mic input leg, untainted source) | **eligible** — shared |
| Firefox screen-sharing *with desktop audio* (input leg on a tainted monitor) | **excluded** |
| EasyEffects / loopback / combine-sink forwarding a mix containing peerspeak | **excluded** |

Where an owner has a tainted input leg and an output leg with **no** resolvable bridge
(§6.1.2), fail closed and exclude the output leg. This only ever engages for owners
actually reading a tainted monitor, so the blast radius is small.

### 6.1.3 ⚠️ Taint must be STICKY — current topology is not enough

**Conceded to Codex in round 6; my "conditional bridge" was correct about topology and
wrong about time.** Taint computed from the *current* graph forgets **buffered audio**.

Failure: an app reads a tainted monitor, buffers or delays (a recorder with a 5 s ring
buffer, a DAW with latency compensation, anything doing lookahead), and then its input leg
untaints or disappears — the user stops sharing to it, the stream corks, the app closes
the capture. A purely topological recompute now sees a clean input, un-taints the owner,
and pixelpass **relinks the output leg while it is still emitting peerspeak audio from
the buffer.** No graph event marks the moment the buffer drains.

**Rule: taint is sticky per owner for the duration of the share.** Once an owner is
tainted it stays tainted until its client/output nodes disappear. Re-eligibility requires
teardown, not a topology change. A timed drain is strictly weaker and would need
measurement to justify; do not ship one in v1.

The cost is that an app which *once* read a tainted monitor stays unshared for the rest of
the share. That is the right trade — it is silence for one app, versus echo for everyone.

### 6.1.4 The transition window is real but bounded

Taint can also *arrive*: a peer joins and speaks, or the AEC loads, mid-share. Recompute
on every graph event and dropping links for newly tainted nodes is **necessary but not
zero-leak**, because a topology change races audio already in flight.

Order of magnitude on this box: a graph quantum is ~`512/48000`–`1024/48000` ⇒ **≈10.6–21.3
ms**, plus main-loop scheduling. App-internal buffers are unbounded in principle. So the
worst case is a sub-quantum-to-tens-of-ms audible sliver at the transition.

⚠️ Mitigating structure: if peerspeak's playback **roots already exist** in the graph and
merely become non-silent (the common case — the AEC and call playback nodes are created at
join, long before anyone speaks), then topological taint has **already** excluded
everything downstream and there is no window at all. The window exists only when a taint
root is *newly created* mid-share. That makes AEC-load-mid-share the case to test.

This means pixelpass must **track Node, Port and Link globals** — not just node
global-add, which is all the existing router does (`pixelpass/src/host/audio.rs:523-584`
subscribes to global-add/global-remove and never binds links). That is a materially
larger component than v3.0 implied.

Ancestry that cannot be resolved (unknown forwarder shape, unresolvable monitor source)
is **not** eligible. That is a deliberate reversal of v3.0's "unrecognized ⇒ fan out":
E2 makes mis-selection cheap *for the user's audio graph*, but it is not cheap for
**echo**, and echo is the whole feature.

### 6.2 The predicate

**Start from all `Stream/Output/Audio` nodes, then exclude:**

| Exclusion | Basis |
| --- | --- |
| `peerspeak.owned` present | §5.1 env tag |
| `pulse.module.id` == the live AEC index | §5.2, exact equality only |
| **tainted via the signal graph** (Link + monitor + conditional owner bridge) | §6.1 — the load-bearing one |
| pixelpass-owned objects, incl. **other hosts'** `pixelpass_capture_*` ancestry | cycle prevention |
| tainted input leg + output leg with no resolvable bridge | §6.1.1, fail closed |
| `port.exclusive` ports, encoded/passthrough streams | fan-out will refuse or corrupt |
| links we already own for that node | idempotence |

- `node.dont-move` **drops out entirely** — fan-out is not a metadata move.
- Links are created **per port**; channel-count and layout mismatches are per-port.

### 6.3 Dynamic graph handling — revalidate, don't fire-and-forget

A node can appear with incomplete ancestry, pass the predicate, get fanned out, and only
*then* receive an inbound link from a filter input or `pixelpass_capture_<pid>.monitor`.
An add-only listener never sees it. Required:

1. An **initial registry sync barrier** — register the listener, call `Core::sync()`, wait
   for `done`, and only then consider the initial pass complete.
   ⚠️ **Know what this barrier is and is not.** PipeWire's `core.h:261` defines sync as a
   *previous-work roundtrip*: because methods and events are ordered, `done` means
   "everything emitted so far has been handled." It is **not** graph quiescence and does
   not promise the graph has stopped changing. Readiness therefore = `done` **plus** "no
   unresolved required Link/Port observations outstanding," with a fail-closed timeout.
2. Candidates stay **pending** until their ports *and* current inbound links are known.
3. **Revalidate immediately before creating each Link**, not just at selection time.
4. On any later link add/remove that makes ancestry unsafe, **drop the owned link
   proxies** for the affected node — retention (§4.2) is what makes revocation possible.

### 6.4 Implementation shape and cost

`pipewire` crate **0.9.2** (`pixelpass/Cargo.toml:48`, `Cargo.lock:4121`).

- **Endpoint props are an OPTIMIZATION, not the correctness path** (settled in round 6).
  Registry `Link` globals carry all four endpoint keys
  (`/usr/include/pipewire-0.3/pipewire/keys.h:256`) and they were observed present on
  every live Link — but the observation was via `pw-dump`, which binds objects, and
  neither the headers nor the crate prove those keys are *always* present in the
  **global-add** props. So: use them when present, and **rely on the bind fallback for
  correctness.** A raw global-add log in the implementation spike settles current
  behaviour, but the fallback must exist regardless.
- **Fallback:** bind that Link, install an info listener, read the endpoint ids off
  `LinkInfoRef` (`pipewire-0.9.2/src/link.rs:153` exposes all four without params), then
  **drop the observer proxy**. Retained proxies are needed only for *our own* fan-out
  links, where dropping is the ownership/lifecycle mechanism (§4.2) — not for external
  links we merely observe.
- **Readiness epoch.** Start fan-out only when: initial `core.sync`/`done` received **and**
  every required Link/Port/Client/Node observer fallback has resolved or hit a fail-closed
  timeout. Then **revalidate in the same graph epoch immediately before creating each
  owned link** — a stale decision from a previous epoch is not a decision.
- **Full recompute per graph event is fine for v1.** Rebuild taint by BFS/DFS from the
  hazard roots, O(V+E). A typical desktop graph is ~40 nodes / ~120 links; this is
  sub-millisecond-class in Rust. No incremental dirty-set in v1; coalesce bursts only if
  logs show churn.

### 6.5 Rejected: the "hardware-sink-only" shortcut

Considered and **rejected**: "fan out only nodes whose output links to a real hardware
sink, excluding anything feeding a virtual/null sink." It is cheaper and it is **unsound
for exactly the case that matters** — measured: `output.loopback-2541-13` links *directly*
to `alsa_output.pci-0000_10_00.6.analog-stereo`. The forwarder's output leg **is** a
hardware-linked node, so the shortcut passes it and leaks the mix. The original app stream
would meanwhile be excluded for feeding the virtual sink — i.e. the shortcut gets both
halves backwards.

It survives only as an optional degraded fallback mode whose stated behaviour is "no
EasyEffects/filter/loopback-routed audio is captured at all."

### 6.6 Foreign/hazard nodes

Treat an unvalidated `echo-cancel-*` output node as an excluded hazard rather than an
eligible stream (§5.4).

## 7. Lifecycle and teardown invariants

### 7.1 ⚠️ The invariant

> **The AEC module must not unload while pixelpass is alive and fanning out.**

If it does, the index pixelpass holds becomes stale, and — because indices are reused
(§5.2 correction 3) — it can alias onto an unrelated future module, silently un-excluding
the real hazard or excluding innocent audio.

### 7.2 ⚠️ VERIFIED DEFECT — implicit-drop order is inverted (not yet fixed)

`ActiveSession` (`peerspeak/src/core/mod.rs:668-691`) declares:

```rust
echo_cancel: Option<EchoCancelGuard>,      // :682
screenshare_host: Option<tokio::process::Child>,   // :685   (kill_on_drop)
```

Rust drops fields in **declaration order**. On the **implicit-drop** path — the command
channels close at `core/mod.rs:1512` (`reliable_rx.recv()` returns `None` → `break`), or
the core loop unwinds — `ActiveSession` is dropped **without** `shutdown()` running. So
`echo_cancel` unloads (a blocking `pactl unload`) *before* `screenshare_host`'s
`kill_on_drop` even fires: **the AEC unloads while pixelpass is still alive.** Exactly the
ordering the invariant forbids. Verified in source.

`shutdown()` (`:694-730`) gets it right — it kills `screenshare_host` at `:699` and drops
`echo_cancel` at `:730` — but only as an emergent property of statement order, which any
refactor can silently invert.

**⚠️ Reordering the fields is NOT sufficient** (round-4 correction). `screenshare_host`
is spawned with `kill_on_drop(true)` (`peerspeak/src/screenshare/mod.rs:402`). Tokio's
`Drop` for such a `Child` *sends* the kill and hands the process to a best-effort orphan
reaper — it does **not** wait, and gives no promptness guarantee, especially if the
runtime is itself shutting down. Explicit `child.kill().await` is SIGKILL **plus wait**;
implicit drop is SIGKILL and move on. So even with `echo_cancel` declared last, the
`pactl unload` can still run while pixelpass is briefly alive.

**Fix, in order of importance:**

1. **Eliminate the implicit path.** At both channel-close `break` sites
   (`core/mod.rs:1516` and `:1532`), explicitly `take()` the session and
   `shutdown().await` it rather than letting it drop. The ordered teardown should be the
   *only* teardown.
2. **Move `echo_cancel` to the last declared field** anyway, with a comment naming the
   invariant — defence in depth for paths 1 does not cover (panics, unwinds).
3. **Last-ditch ordering in the drop path**: a wrapper whose `Drop` does `start_kill` +
   a bounded `try_wait` loop on the host before the AEC guard unloads.

**D4 regression guard** (concrete, replacing v3.0's vague "field-order assertion"): a
test using a **fake host** and **fake echo-cancel guard** that each record a timestamped
event into a shared slot on teardown, asserting `host kill+wait completed` strictly
precedes `echo unload` — plus a core-loop test that closes the command channel and proves
`shutdown()` actually ran. A field-order assertion alone does not test the behaviour that
matters.

### 7.3 Construction paths

There is exactly **one** `echo_cancel::enable` call site (`core/mod.rs:1850`, at session
join), the guard moves into `ActiveSession`, and there is exactly **one** explicit drop
(`:730`). **No mid-call AEC reload path exists.** This is what downgraded Codex's demanded
QUIESCE/SET_AEC/RESUME two-process epoch protocol to:

> **P1-impact latent hazard, currently unreachable in normal operation.** Required now:
> encode teardown ordering for explicit *and* implicit destruction, prevent or review
> additional AEC construction paths, fail closed if identity is missing or revoked.
> **An epoch protocol becomes required if and only if hot AEC reload is added.**

Anyone adding a second `enable` site, or any hot-reload, re-opens that requirement.

### 7.4 Graceful stop is still owed

**Stop Share is currently SIGKILL** (`peerspeak/src/core/mod.rs:699` + `:3480`, Tokio
`Child::kill()`). Under Option C this no longer strands the user's desktop audio — the
links die with the connection, which is the point. But it still leaks:

- The capture null sink is **pactl-loaded and pipewire-pulse-owned**
  (`pixelpass/src/host/audio.rs:69`), so **Stop Share leaks one null-sink module every
  time.** This is true today, independent of this feature.

Required:
1. A graceful control path in peerspeak: **SIGINT** (not SIGTERM — pixelpass installs
   only `tokio::signal::ctrl_c()`, `pixelpass/src/common/signal.rs:6`), bounded wait,
   SIGKILL fallback.
2. The capture sink should become **connection-owned** rather than pactl-owned, so it
   shares the links' death-with-the-process property.
3. `--repair` (`pixelpass/src/repair.rs:15-63`) extended to this mode, and safe with a
   second live host.

## 8. IPC: getting the index to pixelpass

pixelpass is a **separate process** and cannot learn the module index on its own.
peerspeak spawns it, so:

- New pixelpass flag: `--exclude-pulse-module <idx>` (naming, §11).
- peerspeak passes `EchoCancelGuard`'s index at spawn. The field is currently private
  with only `source_name()` / `sink_name()` accessors — add `module_index()`.
- If the AEC is **off**, the flag is absent and pixelpass must not invent an exclusion.
- If the AEC is **on** but the flag is absent, that is a bug in peerspeak; pixelpass
  cannot detect it. ⇒ peerspeak should pass an explicit `--aec=off|<idx>` rather than
  "flag present or not," so pixelpass can distinguish "no AEC" from "someone forgot."
  **Open decision D5 (§13).**
- Because no hot-reload path exists (§7.3), spawn-time argv is sufficient today. A
  runtime channel is required only alongside hot reload.

## 9. What is proven, and what is not

### 9.1 Proven by measurement on this machine (PipeWire 1.6.8 / WirePlumber 0.5.15)

- Fan-out carries full-level audio (−24.1 dB, matching the speaker monitor) for **paplay,
  mpv, VLC**; the app keeps its speaker link.
- WirePlumber does **not** reap foreign links across default-sink switch, switch-back,
  suspend, resume, or 100 s steady state.
- Non-lingering links die on **SIGKILL** of the owner.
- **E2:** destroying the capture sink mid-share left the app playing to speakers
  undisturbed.
- `peerspeak.owned=1` lands on paplay/mpv/VLC nodes via `PULSE_PROP`.
- `pulse.module.id` on all four AEC nodes == the index pactl returned.
- Naive fan-out of the AEC leg leaks remote audio at ≈desktop level; exact exclusion sits
  at the control floor.

### 9.2 ⚠️ Wording discipline — three overclaims already made, do not make a fourth

The correct statement of the exclusion result is:

> *"No incremental 1500 Hz energy was detectable above the control floor at the analysis
> resolution in this steady-state run."*

**Not** "absent," **not** "conclusive." What survives is the gross-leak distinction: the
naive arm copies the probe at ≈desktop level, exact exclusion does not. Likewise, equal
`mean_volume` on a sine proves **signal presence, not fidelity** (blind to xruns, drift,
dropouts, channel swap, quantum change), and "copy semantics" is a **topology** result —
it shows the tested link operation did not move the tested Pulse stream, and says nothing
about gain, latency, continuity, or native clients.

The rig is additionally blind to: startup/teardown/cork/relink transients (hidden by
whole-file averaging), broadband and out-of-band leakage, level-dependent and nonlinear
products, and nodes created after enumeration.

**Minimum rig upgrade before shipping:** two orthogonal PN/MLS probes + windowed
per-channel normalized cross-correlation, reporting max per-window correlation, plus xrun
telemetry.

### 9.3 Untested — carried forward, each one a real risk

AEC reload mid-share · two concurrent AEC instances · two concurrent capture sinks · the
**native** PipeWire AEC module · a native-PipeWire (non-Pulse) app · pixelpass's own
null-sink + loopback present simultaneously · the full GStreamer/AAC/network path ·
fidelity (needs broadband source + correlation + xrun telemetry) · PipeWire daemon restart
· quantum/latency perturbation from the second link · sample-rate mismatch, surround,
IEC958 passthrough · browser and game stream shapes.

## 10. Prerequisite fixes (split out, land before the feature)

Both reviews agree these are their own tasks, not part of this feature.

1. **`object.serial` u32 truncation** — 64-bit in PipeWire, parsed as `u32` at
   `pixelpass/src/host/audio.rs:534-540`. Global IDs are reused; serial is the recommended
   stable identifier. Must be fixed before the router grows.
2. **`@DEFAULT_SINK@` resolved once at module load** (`audio.rs:139-151`), no
   default-metadata listener — the user's `Ctrl+Meta+F` / `Ctrl+Meta+S` output-switch
   hotkeys strand the local monitor mid-share. **This already hurts per-app mode today.**
   Option C does not need the local monitor, so this is decoupled from the feature — but
   it is a live bug.
3. **`try_flush` records intent, not success** (`audio.rs:642-655`): it ignores
   `Metadata::set_property`'s return and appends every pending ID to `routed_node_ids`.
   Any status surfaced to the user inherits that dishonesty. Option C's link-state
   `ACTIVE` check (§4.2) is the honest replacement for the new mode; the old path should
   be fixed or deleted.
4. **Graceful stop + connection-owned capture sink** (§7.4).
5. **Drop-order fix + regression guard** (§7.2).

## 11. Naming — the user's call

Placeholders throughout. pixelpass mode: `--audio-mode=desktop-shared` /
`--audio-mode=desktop-excluding`. Exclusion flag: `--exclude-pulse-module <idx>` (or
`--aec <idx|off>`, §8). Do **not** surface `--exclude-pid` — it names an unreliable
mechanism and reads like process control.

peerspeak picker wording: something like "System audio except peerspeak," noting that
call and watched-share playback are excluded. Whether "All system audio" stays alongside
it (with a stated echo risk) or is replaced when the capability is present is a product
decision.

## 12. Testing

Pure, unit-testable seams with PipeWire at the edges, per house style:

- `fn eligibility(node_props: &Props, ctx: &ExclusionCtx) -> Eligibility` with
  `NotEligible { reason }` so status can explain itself. Cases: `peerspeak.owned` present;
  `pulse.module.id` == live index; `pulse.module.id` present but **different** (must be
  ELIGIBLE — correction 2); `pulse.module.id` absent; a pixelpass-owned node; capture-sink
  ancestry; `port.exclusive`; passthrough; non-`Stream/Output/Audio`; missing props
  entirely; the capture sink itself; a foreign `echo-cancel-*` group.
- `pulse.module.id` parsing: JSON number **and** string forms, values > `u32::MAX`,
  absent, malformed.
- AEC-identity validation state machine: `NotConfigured` / `Validated` / `Revoked`, and
  the assertion that `Revoked` stops fan-out.
- Pure capability-probe parsing (§13 D2).
- Link bookkeeping: per-port link set, "captured" only at all-`ACTIVE`, idempotent
  re-enumeration, proxy retention/drop.

Field tests — the only thing that can prove a viewer does not hear themselves:

1. Sharer in a call while sharing, **AEC on and AEC off**.
2. Sharer simultaneously *viewing* another share while sharing.
3. Lifecycle, each separately: Stop button, room leave, UI crash, pixelpass panic, SIGINT,
   SIGTERM, SIGKILL, last-viewer disconnect, pipewire-pulse restart, PipeWire daemon
   restart, `pixelpass --repair`.
4. Output-device switch mid-share via the real hotkey scripts.
5. Two concurrent hosts (second pixelpass CLI, second peerspeak instance).
6. A notification sound firing mid-share.
7. An app that **starts playing after** the share began (dynamic node).
8. Sample-rate / channel / passthrough behaviour on real sinks; CPU cost.

⚠️ **Test-rig discipline** (each of these already produced a wrong result once):
filter on `media.class` **first** when selecting nodes with `jq` — `media.name` also
matches the Client object; **never** filter stderr out of a measurement run; **verify the
link exists in the graph** before measuring; reproduce over the transport production
actually uses (`parec -d <sink>.monitor`, not `pw-record --target <sink-node-id>`, which
reads −91 dB silence).

## 13. Decisions — resolved in round 4

Both reviewers agree on all seven. Recorded as decided; reopen only with new evidence.

- **D1 — fail closed, but only after a bounded validation wait** (§5.3). Never "share
  anyway, warn" as the default. ✅
- **D2 — a versioned machine-readable capability response or bitset.** The
  `pixelpass --help` substring probe (`src/screenshare/mod.rs:245-278`, invoked at
  `src/core/mod.rs:3368`) stays only as a compatibility fallback, and this mode must
  **not** overload `app_audio_supported: bool` — strict per-app and desktop-excluding are
  independent capabilities. ✅
- **D3 — warn and exclude foreign `echo-cancel-*` nodes** for the first implementation;
  never fan them out. ✅
- **D4 — explicit async shutdown + a fake-resource ordering test** (§7.2). A field-order
  assertion alone is inadequate. ✅
- **D5 — `--aec=off|pulse-module:<idx>`, always passed.** Absence of the flag is not a
  protocol state (§8). ✅
- **D6 — land §10 items 1, 4 and 5 first**, plus the new graph-ancestry predicate (§6.1).
  Items 2 and 3 are real per-app debt but do not block Option C unless the new mode reuses
  those code paths. ✅
- **D7 — no materially simpler design exists** that still meets Joe's ask. The available
  simplification is to *narrow v1 scope*, not to change architecture. ✅

## 14. Readiness — 🔴 BLOCKED (round 4), unblock set

v3.0 was reviewed and blocked; v3.1 (this revision) applies the three required rewrites.
The minimum unblock set was:

1. **§6 rewritten around transitive graph eligibility + dynamic link revalidation** —
   applied in §6.1–6.3. This is the round's biggest finding: eligibility is a graph
   property, and **EasyEffects is installed on the target machine**, so the leak is
   reachable, not theoretical.
2. **§5.3 rewritten as a bounded AEC validation state machine with explicit `--aec`** —
   applied.
3. **§7.2's field-order fix replaced with explicit shutdown + a real regression guard** —
   applied.

**Next step: a fifth round re-reviewing v3.1**, focused on whether the graph-ancestry
predicate in §6.1 is actually implementable against the `pipewire` crate's Link/Port
globals, and on the cost of the sync barrier in §6.3. Nothing here is approved for merge.